Nitriding Treatment Method and Nitriding Treatment Apparatus

ABSTRACT

The present invention provides a nitriding treatment method for forming a compound layer of ε phase (Fe 2-3 N) and γ′ phase (Fe 4 N) iron nitride excellent in wear durability in a steel material, from which a sliding member is formed, by short treatment, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load. The nitriding treatment method of the present invention includes heating a sliding member made of a steel material at a temperature of 600° C. to 700° C. for a time of 1 to 25 minutes under an atmosphere of nitriding gas through high frequency induction heating or resistive heating, to form a compound layer of ε phase (Fe 2-3 N) and γ′ phase (Fe 4 N) iron nitride, the compound layer having a nitrogen content of higher than 4.5%, in a surface layer portion of the sliding member.

TECHNICAL FIELD

The present invention relates to a nitriding treatment method and a nitriding treatment apparatus.

BACKGROUND ART

Wear such as “galling” and “exfoliation” may occur at surfaces of sliding members (each hereinafter called a “sliding member”) made of a steel material and used under severe wear environments in pistons of hydraulic pumps, cylinders, valves, or injection nozzles of injection molding machines, or the like.

Wear decreases the service life of a sliding member. There is hence a need to improve the wear durability (wear strength) of the sliding member. There is also a need to treat the sliding member in a short time, so that its manufacturing cost is reduced and its one-piece flow manufacturing is realized.

For steel materials to be used in automobile members, building members, and the like, on the other hand, strength, workability, weldability, and the like are needed.

As background art in the technical field associated with such steel materials, reference can be made to JP-2005-14632-A (Patent Document 1). Patent Document 1 describes a method for manufacturing a steel material having a microstructure, the method including holding the steel material at a temperature of 550° C. or higher for a time of 1 second or longer in an atmosphere containing ammonia at 0.5% or higher, so that 0.05% or more of nitrogen in terms of percent by mass is incorporated.

As a nitriding treatment method, reference can be made to JP-2017-137547-A (Patent Document 2). Patent Document 2 describes a nitriding treatment method for allowing nitrogen to penetrate and diffuse in a work made of a steel material, the method including applying induction heating to the work at such a frequency that a current penetration depth of 2 mm or greater is reached, and blowing ammonia gas against a surface of the work.

PRIOR ART DOCUMENT Patent Documents

Patent Document 1: JP-2005-146321-A

Patent Document 2: JP-2017-137547-A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Patent Document 1 describes the method for manufacturing a steel material in which 0.05% or more of nitrogen in terms of percent by mass is incorporated, and Patent Document 2 describes the nitriding treatment method including an application of induction heating to a work and blowing of ammonia gas against a surface of the work.

However, none of Patent Documents 1 and 2 contain any description about a compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride.

Further, neither Patent Document 1 nor 2 states that, for an improvement in the wear durability of a sliding member, there is a need to form a compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer having such a high nitrogen concentration as exceeding 4.5% in nitrogen content, in a steel material from which the sliding member is formed. In particular, Patent Document 1 states that the upper limit of the nitrogen content is set at 4% for a potential impairment of ductility, and Patent Document 2 states that the nitrogen content is approximately 0.3% in a surface layer portion.

The present invention therefore provides a nitriding treatment method and a nitriding treatment apparatus for forming a compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability in a steel material, from which a sliding member is formed, by short treatment, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load.

Means for Solving the Problems

To solve the above-described problems, a nitriding treatment method of the present invention includes heating a sliding member made of a steel material at a temperature of 600° C. to 700° C. for a time of 1 to 25 minutes under an atmosphere of nitriding gas through high frequency induction heating or resistive heating, to form a compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer having a nitrogen content of higher than 4.5%, in a surface layer portion of the sliding member.

Also, to solve the above-described problems, a nitriding treatment apparatus of the present invention includes a nitriding chamber in which a sliding member made of a steel material is to be placed, a high frequency heating coil disposed in the nitriding chamber to subject the sliding member to high frequency heating, a vacuum pump that evacuates the nitriding chamber, a nitriding gas cylinder that supplies nitriding gas to the nitriding chamber, a nitrogen gas cylinder that supplies nitrogen gas to the nitriding chamber, a high frequency power source connected to the high frequency heating coil to energize the high frequency heating coil, and a forced convection generating unit disposed in the nitriding chamber to generate forced convection in the nitriding gas.

Advantage of the Invention

According to the present invention, it is possible to provide a nitriding treatment method and a nitriding treatment apparatus for forming a compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability in a steel material, from which a sliding member is formed, by short treatment, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load.

It is to be noted that problems, elements, and advantageous effects other than those described above will become apparent from the details of Examples to be described subsequently herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a nitriding treatment apparatus which will be described in Example 1.

FIG. 2 is a diagram showing a cross-sectional structure of a test specimen 1 subjected to nitriding treatment in Example 1.

FIG. 3 is a diagram showing results of EBSD (electron backscatter diffractometry) analysis of the test specimen 1 subjected to the nitriding treatment in Example 1.

FIG. 4 is a diagram showing a hardness distribution in a depth direction of the test specimen 1 subjected to the nitriding treatment in Example 1.

FIG. 5 is a diagram showing results of EPMA (electron probe microanalyzer) analysis of the test specimen 1 subjected to the nitriding treatment in Example 1.

FIG. 6 is a diagram showing a nitriding treatment apparatus which will be described in Example 3.

FIG. 7 is a diagram showing a cross-sectional structure of a test specimen 1 subjected to nitriding treatment in Example 3.

FIG. 8 is a diagram showing a nitriding treatment apparatus which will be described in Example 4.

FIG. 9 is a diagram showing a nitriding treatment apparatus which will be described in Example 5.

MODES FOR CARRYING OUT THE INVENTION

Using the drawings, Examples of the present invention will hereinafter be described. It is to be noted that substantially the same or similar configurations are denoted by the same reference characters, and any overlapping description may be omitted.

EXAMPLE 1

A nitriding treatment apparatus of Example 1 will be described first.

FIG. 1 is a diagram showing the nitriding treatment apparatus of Example 1.

The nitriding treatment apparatus of Example 1 includes a nitriding chamber 2 in which a test specimen 1 is to be placed, a high frequency heating coil 3 disposed in the nitriding chamber 2 to subject the test specimen 1 to high frequency heating, a vacuum pump 4 that evacuates the nitriding chamber 2, an ammonia gas (nitriding gas) cylinder 5 that supplies ammonia gas (nitriding gas) 6 of 100% concentration to the nitriding chamber 2, a nitrogen gas cylinder 8 that supplies nitrogen gas 9 to the nitriding chamber 2, and a high frequency power source 7 connected to the high frequency heating coil 3 to energize the high frequency heating coil 3.

It is to be noted that, in Example 1, the ammonia gas 6 of 100% concentration is used. Ammonia gas 6 of another concentration can also be used if a necessary amount (concentration) of the ammonia gas 6 can be supplied to a surface of the test specimen 1.

Further, the test specimen 1 is used as a sliding member in Example 1. Using, as the material of the test specimen 1, a general chromium molybdenum steel material containing, in terms of percent by mass, C: 0.33% to 0.38%, Si: 0.15% to 0.35%, Mn: 0.60% to 0.90%, P: 0.03% or less, S: 0.03% or less, Ni: 0.25% or less, Cr: 0.90% to 1.20%, and Mo: 0.15% to 0.30%, a rod-shaped test specimen 1 of 10 mm diameter and 50 mm length is formed.

A nitriding treatment method of Example 1 will next be described.

Step (1): Place the test specimen 1 in the nitriding chamber 2.

Step (2): Using the vacuum pump 4, evacuate the nitriding chamber 2 to a pressure of approximately 0.5 Pa.

Step (3): Supply the ammonia gas 6 from the ammonia gas cylinder 5 to the nitriding chamber 2 to restore the pressure of the nitriding chamber 2 to 8×10⁴ Pa. It is to be noted that, as the concentration of the ammonia gas 6 is determined by this pressure, this pressure may preferably be restored to rise to 2×10⁴ Pa or higher.

Step (4): Hold (heat) the test specimen 1 at a temperature of 630° C. for a time of 3 minutes by the high frequency power source 7 connected to the high frequency heating coil 3. Here, natural convection of the ammonia gas 6 occurs along the surface of the test specimen 1. Alternatively, forced convection of the ammonia gas 6 is generated here along the surface of the test specimen 1. In this manner, the nitriding potential (the partial pressure of ammonia/the partial pressure of hydrogen) at a surface layer portion (in a vicinity of the surface) of the test specimen 1 increases, so that nitriding treatment can be performed with a high nitriding potential.

To the surface of the test specimen 1, the necessary amount (concentration) of the ammonia gas 6 is supplied via natural convection or forced convection. It is to be noted that the supply of the ammonia gas 6 to the surface of the test specimen 1 via forced convection is preferred to perform the nitriding treatment with a still higher nitriding potential.

Described specifically, the ammonia gas 6 is heated in a vicinity of the test specimen 1. By a temperature difference between the ammonia gas 6 heated to a temperature (600° C. to 700° C.), which contributes to a nitriding reaction, and the cold unreacted ammonia gas 6 existing in a space remote from the test specimen 1, natural convection occurs in the ammonia gas 6. In this manner, the unreacted ammonia gas 6 is supplied to the surface of the test specimen 1, so that the nitriding treatment is performed with the high nitriding potential. It is to be noted that forced convection is preferably caused to occur along the surface of the test specimen 1 to supply the ammonia gas 6. In this manner, the nitriding treatment can be performed with the still higher nitriding potential.

Step (5): Using the vacuum pump 4, evacuate the ammonia 6 in the nitriding chamber 2.

Step (6): Cool the test specimen 1 with the nitrogen chamber 2 kept in nitrogen purged with the nitrogen gas 9 supplied from the nitrogen gas cylinder 8.

In summary, the nitriding treatment method of Example 1 holds (heats) a sliding member (the test specimen 1) made of a steel material at a temperature of 600° C. to 700° C. for a time of 1 to 25 minutes under an atmosphere of nitriding gas (the ammonia gas 6) through high frequency induction heating or resistive heating while generating convection of the nitriding gas along the surface of the sliding member and supplying unreacted nitriding gas, and after this nitriding treatment, cools the sliding member.

As a result, a compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer having a high nitrogen concentration exceeding 4.5% (but lower than 11%) in nitrogen content, is formed in the surface layer portion of the sliding member.

In Example 1, the compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer having the high nitrogen concentration exceeding 4.5% in nitrogen content, is formed in the surface layer portion of the test specimen 1 (the steel material forming the sliding member) by heating the test specimen 1 at the temperature of 600° C. to 700° C. for the short time (1 to 25 minutes) under the atmosphere of the nitriding gas (the ammonia gas 6) through high frequency induction heating or resistive heating as described above.

It is to be noted that, specifically, the compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer having the high nitrogen concentration exceeding 4.5% in nitrogen content, is formed from ε phase, ε phase+γ′ phase, γ′ phase+ε phase, and γ′ phase.

In the manner described above, the compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability can be formed in the surface layer portion of the sliding member by short (1 to 25 minutes) treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load.

Owing to the short treatment suited for one-piece flow manufacturing, it is also possible to reduce the manufacturing cost of the sliding member and, when sliding members are mass produced, to make uniform the quality of each sliding member.

It is to be noted that the realization of one-piece flow manufacturing needs to form the compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability in the short time of 1 to 25 minutes although this depends on the shape and size of the sliding member.

It is also to be noted that the nitriding treatment and the cooling can be performed using the nitriding chamber 2 in common but the nitriding treatment and the cooling can also be continuously performed by performing the nitriding treatment with the nitriding chamber 2 and performing the cooling with another vessel.

It is to be understood that the expression “excellent in wear durability” means to be excellent in sliding characteristics and to be small in friction coefficient. It is also to be understood that the term “wear durability” is characteristics required especially for sliding members.

A cross-sectional structure of the test specimen 1 subjected to the nitriding treatment in Example 1 will be described next.

FIG. 2 is a diagram showing the cross-sectional structure of the test specimen 1 subjected to the nitriding treatment in Example 1.

FIG. 2 shows results obtained by slicing the test specimen 1 and observing the cross-sectional structure of an outer peripheral portion of a sliced surface. According to FIG. 2 , it is understood that the iron nitride compound layer 10 of approximately 3.0 to 4.0 μm thickness is formed in the surface layer portion of the test specimen 1. It is to be noted that an outermost peripheral portion is a resin layer.

Results of EBSD analysis of the test specimen 1 subjected to the nitriding treatment in Example 1 will be described next.

FIG. 3 is a diagram showing the results of the EBSD analysis of the test specimen 1 subjected to the nitriding treatment in Example 1.

FIG. 3 shows the results of EBSD analysis of an outer peripheral portion of the test specimen 1, a cross-sectional structure of which was observed. According to FIG. 3 , it is understood that an ε phase (Fe₂₋₃N) 11, a γ′ phase (Fe₄N) 12, a diffused layer (α phase+γ′ phase) 13, and an α phase 14 are formed in the surface layer portion of the test specimen 1.

According to FIG. 3 , it is also understood that the compound layer 10 of the iron nitride of the phase (Fe₂₋₃N) 11 and the γ′ phase (Fe₄N) 12, the compound layer 10 having the high nitrogen concentration exceeding 4.5% in nitrogen content, is formed in the surface layer portion of the test specimen 1.

A hardness distribution in a depth direction of the test specimen 1 subjected to the nitriding treatment in Example 1 will be described next.

FIG. 4 is a diagram showing the hardness distribution in the depth direction of the test specimen 1 subjected to the nitriding treatment in Example 1.

FIG. 4 shows a correlation between the distance from the surface (the depth direction) and the Vickers hardness.

According to FIG. 4 , it is understood that the surface layer portion of the test specimen 1 has been hardened by the compound layer 10 of the iron nitride and the hardness continuously decreases as the distance increases from the surface toward the inside.

Results of EPMA analysis of the test specimen 1 subjected to the nitriding treatment in Example 1 will be described next.

FIG. 5 is a diagram showing the results of the EPMA analysis of the test specimen 1 subjected to the nitriding treatment in Example 1.

FIG. 5 shows a correlation between the distance from the surface (the depth direction) and the nitrogen concentration (nitrogen content). According to FIG. 5 , it is understood that the surface layer portion of the test specimen 1 has been increased in nitrogen concentration by the compound layer 10 of the iron nitride and the nitrogen concentration decreases as the distance increases from the surface toward the inside.

As described above, it is appreciated that, according to Example 1, a compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability can be formed in the surface layer portion of the test specimen 1 by short treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load.

Further, a comparison was made between a nitriding rate in a method A, in which the ammonia gas 6 was heated in its entirety as in furnace heating, and a nitriding rate in a method B, in which the ammonia gas 6 was heated in a vicinity of a test specimen 1 as in the high frequency induction heating using the high frequency heating coil 3.

It is to be noted that the heating temperature, the holding time, and the concentration of the ammonia gas 6 were set the same in the method A and the method B.

As a result of the comparison, the time required for the formation of the compound layer 10 of the iron nitride of the phase (Fe₂₋₃N) 11 and the γ′ phase (Fe₄N) 12, the compound layer 10 having the same thickness and the high nitrogen concentration exceeding 4.5% in nitrogen content, in the surface layer portion of the test specimen 1 was approximately 1/40 in the method B relative to the method A.

It is therefore understood that the method B can form a compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability in a surface layer portion of a test specimen 1 by short treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load.

Further, a test specimen A1 in the shape of a round rod of 10 mm diameter and 10 mm length was formed by performing nitriding treatment at a temperature of 580° C. for a time of 3 hours through furnace heating. In addition, another test specimen B1 in the shape of a round rod of 10 mm diameter and 10 mm length was formed under the same conditions as in Example 1.

A reciprocating sliding wear test was performed on the test specimen A1 and the test specimen B1. Compared with the test specimen A1, the test specimen B1 was equal or higher in friction coefficient, and was also equal or longer in time until adhesion.

Accordingly, it has been confirmed that a sliding member to be formed under the conditions of Example 1 will have the wear durability for sliding members.

A hydraulic pump piston A2 of 30 mm diameter and 10 mm length was also formed by performing nitriding treatment at a temperature of 580° C. for a time of 3 hours through furnace heating. In addition, another hydraulic pump B2 of 30 mm diameter and 10 mm length was formed under the same conditions as in Example 1.

The hydraulic pump piston A2 and the hydraulic pump piston B2 were each assembled in a hydraulic pump, and a durability test was performed. Compared with the hydraulic pump piston A2, the hydraulic pump piston B2 had an equal or longer service life.

Accordingly, it has been confirmed that a sliding member to be formed under the conditions of Example 1 will have the wear durability for sliding members.

As described above, it is understood that, by performing the nitriding treatment of Example 1 on a sliding member, a compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability can be formed in a surface layer portion of the sliding member by short treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load.

EXAMPLE 2

In Example 2, nitriding treatment was performed by changing the temperature and time from Example 1. More specifically, nitriding treatment was performed in Example 2 by changing the temperature and time in Step (4) of the nitriding treatment method of Example 1.

In Example 2, nitriding treatment was performed under four sets of conditions: (a) 600° C.×1 minute, (b) 600° C.×25 minutes, (c) 700° C.×1 minute, and (d) 700° C.×25 minutes.

Although some differences were observed in thickness and nitrogen content among the resulting iron nitride compound layers 10, it is understood that, under each set of conditions, the compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer 10 having a high nitrogen concentration exceeding 4.5% in nitrogen content, was formed in the surface layer portion of the test specimen 1.

In Example 1 and Example 2, the test specimens 1 were each heated under the atmosphere of the ammonia gas 6, at the temperature of 600° C. to 700° C., for the time of 1 to 25 minutes, through high frequency induction heating or resistive heating, as described above.

Accordingly, it is understood that, under any conditions described above, a compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability can be formed in a surface layer portion of a test specimen 1 by short treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load.

EXAMPLE 3

A nitriding treatment apparatus of Example 3 will be described next.

FIG. 6 is a diagram showing the nitriding treatment apparatus of Example 3.

Compared with the nitriding treatment apparatus described in Example 1, the nitriding treatment apparatus of Example 3 is different in having a stirrer 15 (forced convection generating unit) that is disposed inside the nitriding chamber 2 and stirs the ammonia gas 6 supplied into the nitriding chamber 2.

The stirrer 15 is disposed in a lower part of the nitriding chamber 2 in Example 3, but may also be disposed in an upper part of the nitriding chamber 2.

By generating forced convection of the ammonia gas 6 with the stirrer 15 thus provided, the unreacted ammonia gas 6 was forcedly supplied to a surface of a test specimen 1, thereby enabling to subject the test specimen 1 to nitriding treatment with a still higher nitriding potential. It is to be noted that the stirring flow rate of the stirrer 15 was set at 100 mL/min.

In addition, a baffle plate (not shown) may also be disposed in a supply line of the ammonia gas 6 to facilitate the generation of the forced convection of the ammonia gas 6 along the surface of the test specimen 1 and also to uniformly supply the ammonia gas 6 to the surface of the test specimen 1. Such a baffle plate enables to supply the unreacted ammonia gas 6 to the surface of the test specimen 1, to provide an improved nitriding rate, and to perform nitriding treatment with a still higher nitriding potential.

A cross-sectional structure of the test specimen 1 subjected to the nitriding treatment in Example 3 will be described next.

FIG. 7 is a diagram showing the cross-sectional structure of the test specimen 1 subjected to the nitriding treatment in Example 3.

FIG. 7 also shows results obtained as in Example 1 by slicing the test specimen 1 and observing the cross-sectional structure of an outer peripheral portion of a sliced surface with the same magnification. According to FIG. 7 , it is understood that an iron nitride compound layer 10 of approximately 5.0 to 6.0 μm thickness (excluding a microporous layer) is formed in a surface layer portion of the test specimen 1.

In the case of an iron nitride compound layer 10 of approximately 3.0 to 4.0 μm thickness, it can therefore be formed in a shorter time than in Example 1.

By forcedly supplying the unreacted ammonia gas 6 to the surface of the test specimen 1 as described above, the nitriding rate was improved, thereby enabling to perform nitriding treatment with a still higher nitriding potential.

In the surface layer portion of the test specimen 1, the compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer 10 having a high nitrogen concentration exceeding 4.5% in nitrogen content, was formed accordingly.

According to Example 3, it is understood that a compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability can be formed in a surface layer portion of a test specimen 1 by short (still shorter) treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load, as appreciated from the foregoing.

Further, a comparison was made between the nitriding rate in the method A, in which the ammonia gas 6 was heated in its entirety as in furnace heating, and a nitriding rate in a method C, in which the ammonia gas 6 was heated in a vicinity of a test specimen 1 as in the high frequency induction heating using the high frequency heating coil 3 and forced convection was generated.

It is to be noted that the heating temperature, the holding time, and the concentration of the ammonia gas 6 were set the same in the method A and the method C.

As a result of the comparison, the time required for the formation of the compound layer 10 of the iron nitride of the phase (Fe₂₋₃N) 11 and the γ′ phase (Fe₄N) 12, the compound layer 10 having the same thickness and the high nitrogen concentration exceeding 4.5% in nitrogen content, in the surface layer portion of the test specimen 1 was approximately 1/50 to 1/60 in the method C relative to the method A.

It is therefore understood that the method C can form a compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability in a surface layer portion of a test specimen 1 by short (still shorter) treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load.

EXAMPLE 4

A nitriding treatment apparatus of Example 4 will be described next.

FIG. 8 is a diagram showing the nitriding treatment apparatus of Example 4.

Compared with the nitriding treatment apparatus described in Example 3, the nitriding treatment apparatus of Example 4 is different in having an up-down drive unit 16 (forced convection generating unit) that is disposed inside the nitriding chamber 2 and moves up and down a test specimen 1 placed in the nitriding chamber 2. In other words, the up-down drive unit 16 is disposed in place of the stirrer 15 in Example 4.

By generating forced convection of the ammonia gas 6 with the up-down drive unit 16 as described above, the unreacted ammonia gas 6 can forcedly be supplied to a surface of a test specimen 1, so that the test specimen 1 can be subjected to nitriding treatment with a still higher nitriding potential.

According to Example 4, a compound layer 10 of phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer 10 having a high nitrogen concentration exceeding 4.5% in nitrogen content, is formed in a surface layer portion of the test specimen 1, although, compared with Example 3, some differences may arise in the thickness and nitrogen content of the iron nitride compound layer 10.

Further, according to Example 4, an iron nitride compound layer 10 is formed with a greater thickness than that in Example 1 in a surface layer portion of a test specimen 1. In other words, in the case of an iron nitride compound layer 10 having the same thickness as that in Example 1, it can be formed in a shorter time than in Example 1.

According to Example 4, it is understood that a compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability can be formed in a surface layer portion of a test specimen 1 by short (still shorter) treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load, as appreciated from the foregoing.

It is to be understood that the up-down drive unit 16 may also perform rotary drive in combination with up-down drive.

EXAMPLE 5

A nitriding treatment apparatus of Example 5 will be described next.

FIG. 9 is a diagram showing the nitriding treatment apparatus of Example 5.

Compared with the nitriding treatment apparatus described in Example 3, the nitriding treatment apparatus of Example 5 is different in having a rotary drive unit 17 (forced convection generating unit) that is disposed inside the nitriding chamber 2 and rotates a test specimen 1 placed in the nitriding chamber 2. In other words, the rotary drive unit 17 is disposed in place of the stirrer 15 in Example 5.

By generating forced convection of the ammonia gas 6 with the rotary drive unit 17 as described above, the unreacted ammonia gas 6 can forcedly be supplied to a surface of a test specimen 1, so that the test specimen 1 can be subjected to nitriding treatment with a still higher nitriding potential.

According to Example 5, a compound layer 10 of phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer 10 having a high nitrogen concentration exceeding 4.5% in nitrogen content, is formed in a surface layer portion of a test specimen 1, although, compared with Example 3, some differences may arise in the thickness and nitrogen content of the iron nitride compound layer 10.

Further, according to Example 5, an iron nitride compound layer 10 is formed with a greater thickness than that in Example 1 in a surface layer portion of a test specimen 1. In other words, in the case of an iron nitride compound layer 10 having the same thickness as that in Example 1, it can be formed in a shorter time than in Example 1.

According to Example 5, it is understood that a compound layer 10 of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride excellent in wear durability can be formed in a surface layer portion of a test specimen 1 by short (still shorter) treatment suited for one-piece flow manufacturing, with a high thermal efficiency, with a reduced amount of used nitriding gas, and with a low environmental load, as appreciated from the foregoing.

It is to be understood that the rotary drive unit 17 may also perform up-down drive in combination with rotary drive.

In addition, a blade (not shown) may be disposed on the rotary drive unit 17. Such a blade enables to forcedly supply the unreacted ammonia gas 6 to the surface of the test specimen 1, to provide an improved nitriding rate, and to perform nitriding treatment with a still higher nitriding potential.

It is to be noted that the present invention is not limited to the above-described Examples and embraces a variety of modifications.

For example, the above-described Examples specifically illustrate the present invention to facilitate its understanding, and the present invention should not be absolutely limited to those including all the elements described.

Further, one or more of the elements of one of the Examples can be replaced by one or more of the elements in another one of the Examples.

Furthermore, one or more of the elements of one of the Examples can be added to the elements of another one of the Examples. Moreover, with respect to one or more of the elements of each Example, the element or elements can be omitted, can be added with one or more of the elements of another Example, or can be replaced by one or more of the elements of another Example.

DESCRIPTION OF REFERENCE CHARACTERS

1: Test specimen, 2: Nitriding chamber, 3: High frequency heating coil, 4: Vacuum pump, 5: Ammonia gas cylinder, 6: Ammonia gas, 7: High frequency power source, 8: Nitrogen gas cylinder, 9: Nitrogen gas, 10: Iron nitride compound layer, 11: ε phase (Fe₂₋₃N), 12: γ′ phase (Fe₄N), 13: Diffused layer, 14: a phase, 15: Stirrer, 16: Up-down drive unit, 17: Rotary drive unit 

1. A nitriding treatment method comprising: heating a sliding member made of a steel material at a temperature of 600° C. to 700° C. for a time of 1 to 25 minutes under an atmosphere of nitriding gas through high frequency induction heating or resistive heating, to form a compound layer of ε phase (Fe₂₋₃N) and γ′ phase (Fe₄N) iron nitride, the compound layer having a nitrogen content of higher than 4.5%, in a surface layer portion of the sliding member.
 2. The nitriding treatment method according to claim 1, wherein, after the sliding member is heated, the sliding member is cooled.
 3. The nitriding treatment method according to claim 1, wherein the nitriding gas is supplied to a surface of the sliding member via natural convection or forced convection.
 4. A nitriding treatment apparatus comprising: a nitriding chamber in which a sliding member made of a steel material is to be placed; a high frequency heating coil disposed in the nitriding chamber to subject the sliding member to high frequency heating; a vacuum pump that evacuates the nitriding chamber; a nitriding gas cylinder that supplies nitriding gas to the nitriding chamber; a nitrogen gas cylinder that supplies nitrogen gas to the nitriding chamber; a high frequency power source connected to the high frequency heating coil to energize the high frequency heating coil; and a forced convection generating unit disposed in the nitriding chamber to generate forced convection in the nitriding gas.
 5. The nitriding treatment apparatus according to claim 4, wherein the forced convection generating unit is a stirrer that stirs the nitriding gas.
 6. The nitriding treatment apparatus according to claim 4, wherein the forced convection generating unit is an up-down drive unit that moves the sliding member up and down.
 7. The nitriding treatment apparatus according to claim 4, wherein the forced convection generating unit is a rotary drive unit that rotates the sliding member. 